1. Field of the Invention
This invention relates generally to crystal oscillators, and more specifically to an improved circuit for reliably assuring that a crystal oscillator will commence oscillating and for decreasing the time required for the crystal oscillator to reach its maximum oscillation amplitude.
2. Description of the Prior Art
Crystal oscillators have long been utilized in devices which require accurate and/or precise clocks, such as microprocessors or the like. A common problem associated with known crystal oscillators, however, has been their failure to commence oscillating at all or within an acceptably short time period. In addition, a large period of time is required for known oscillators to attain their maximum oscillation amplitude once the circuit power supply is turned on, necessitating a large period of time for start-up of the devices in which they are utilized.
In MOS technology, crystal oscillators are typically formed in a circuit such as that illustrated in FIG. 1. A crystal element 30 is provided which is connected in parallel to a high gain CMOS inverter 32 formed by a P-channel transistor 42 and an N-channel transistor 44. Stabilizing capacitors 34 and 36 are provided which are connected to ground and respectively to the input 38 and the output 40 of the inverter 32. In operation, when the circuit supply voltage V.sub.cc is turned on, the transistor 42 will be on, generating a voltage V.sub.out at the inverter output 40. Volta V.sub.out then charges up the capacitor 36 and develops a voltage at the crystal 30, causing it to begin oscillating at a low amplitude. This generates a voltage V.sub.in at the input 38 of the inverter and charges capacitor 34. The increasing voltage V.sub.in slowly causes the inverter to move to its high gain region, thereby allowing the amplitude of the oscillations of the crystal to increase. Repeated feedback in this manner establishes a voltage over time at the crystal 30 as illustrated in FIG. 2.
Whether crystal 30 will commence oscillation depends on several factors, including the rise time and the shape of the power supply waveform. In addition, the absence of noise can result in start-up failure. In most circuits, noise is undesirable. In the circuit of FIG. 1, however, noise at the input to the inverter actually aids in achieving start-up.
If crystal 30 commences oscillation, several factors affect the time required for the oscillator to attain maximum amplitude oscillations. These factors include, but are not limited to, the gain of the inverter 32 and the noise level passing through the inverter 32. A high gain inverter will amplify existing noise more to achieve maximum amplitude variation at the output faster than will a low gain inverter. In addition, a high level of noise will require less amplification to achieve the necessary amplitude variation to support oscillation than will a lesser level of noise.
The known inverter 32 has a voltage characteristic as illustrated by line 45 of FIG. 3. The slope of this curve, d V.sub.out /d V.sub.in, is the voltage gain of the inverter. The desirable goal of attaining maximum gain (and thus minimum start-up time for the oscillator) is obtainable at the inverter when V.sub.out is equal to V.sub.in, and may be derived from the circuit by placing a resistor element 46 in parallel with the inverter 32 to aid in the charging of the capacitor 34. Line 48 plotted on the voltage characteristic of FIG. 3 represents the relationshi V.sub.in =V.sub.out which results when resistor element 46 is connected in parallel to the inverter 32. The resistor element 46 biases the inverter to operate at the maximum gain point 50 of curve 45, thereby reducing the time required by the oscillator circuit to reach maximum voltage amplitude.
The start-up time of the circuit may be minimized by charging the capacitor 34 as quickly as possible to place the inverter in its high gain region. This may be accomplished by choosing the value of resistance of the resistor element 46 to be as small as possible, thus allowing the voltage V.sub.out to charge up the capacitor 34 quickly.
A major constraint has existed in the past, however, on the minimum value of resistance which may be chosen for the resistor element 46. Oscillating crystals characteristically pass signals only of select frequencies and prevent passage of all other frequencies. The designer must thus attempt to eliminate all non-select frequencies from the circuit such that they do not degrade the strength of the signal of the select frequency. When the resistor element 46 is added to the circuit to achieve high gain, however, an additional feedback path 52 is created between the input 38 and the output 40 of the inverter 32 which bypasses the crystal 30. Noise of undesirable frequencies present at the output 40 of the inverter 32 may thus travel through this additional feedback path 52 to the input 38 of the inverter 32 and be amplified, therby degrading the overall performance of the oscillator. Designers have thus in the past selected the resistor element 46 with a relatively high value of resistance in order to prevent most frequencies of noise from traveling through the feedback path 52, thereby avoiding degradation of the performance of the oscillator.
Designers have thus previously had to balance the desire for noise-free operation during oscillation by providing a high value of resistance for resistor element 46 with the desire for a fast start-up time by providing a resistor element having a small value of resistance. Typically, the desire to achieve noise-free oscillation has been dominant, resulting in less than desirable start-up times for oscillator circuits in the past.
Known crystal oscillator circuits have thus not only failed to commence oscillation in the past, but in addition have contained constraints upon the speed at which maximum oscillation amplitude could be obtained if oscillation could be commenced.